NA6 - Misfire, Poor idle, Rich, Black smoke.

[gazNA6CE]

Hi there, Im not new but this is my first topic thread

I need opinions on a fault that I am experiencing with my NA6. It still has the B6 engine fitted in it which apart from a alloy radiator isn't modified. On cold start there is a misfire but once the engine has reached N.O.T the misfire disappears. If i let the car idle for a few minutes after driving or after a hot start the revs start to fluctuate and it blows black smoke from the exhaust with the throttle slightly depressed but then it clears up and returns back to normal. Also smells a tad on the rich side too..

The car has new leads and plugs. I'm thinking O2 sensor, MAF sensor or ISCV.

What do you think? Cheers.

Oh and it has a newish non genuine CTS fitted to it which I have doubts about.

[gslender]

The coolant system is integrated with idle control in two places. At the ISCV under the TB and on the Intake Manifold itself as a fast idle valve.

Check that both of these are flowing coolant and that the fast idle valve is working. You can take it off the manifold and visually test it in hot water. Check that it is correctly open when cold and closed when hot near boiling.

G

[koop21]

Gaz, your not wrong to think ISCV they do get very dirty over time and can hinder the move of the valve during hot idle.

Check it this write-up here

[gazNA6CE]

Thanks for the quick advice, Ill have a look at ISCV system. Ill let you know what I find.

[manga_blue]

Gaz, the write-up on miata.net is a good one but it misses out on mentioning cooling system service. I've seen a couple of these cars now where accumulated gunk in the coolant and flaking rubber inside the small hoses completely stopped any coolant from flowing to the ISCV. When you check the ISCV just make sure that all the little hoses are in good nick and the coolant is fresh and clean.

[gazNA6CE]

Hi there

Just giving you an update on what's happening, I removed and cleaned the ISCV then reinstalled it. The problem is less frequent and the miss fire upon cold start up is gone.

I'm still going to replace the ISCV. Cheers for all the advice.